A thermal recording system for obtaining recorded images simultaneously with application of input signals is widely used in facsimiles, computer terminal printers, and printers for measuring equipment because the apparatus used in the system is relatively easy to handle and inexpensive and is of low noise.
The recording medium commonly employed in the thermal recording system is a so-called color formation type heat-sensitive recording material having a recording layer which undergoes physical and chemical changes on heating to cause color formation. The recording medium of this type, however, is liable to induce undesired color formation during the production or storage thereof. Moreover, the image formed thereon has poor storage stability. For example, the image undergoes fading when brought into contact with organic solvents or chemicals.
In order to overcome these problems, it has been proposed to use a recording medium utilizing a self-colored coloring material in place of the abovedescribed color formation type heat-sensitive recording medium. For example, JP-A-51-15446 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a recording system in which a support, such as paper and a polymer film, coated with a coloring material which is solid or semi-solid at room temperature is superposed on a recording material (image-receiving sheet) in such a manner that the coloring material comes into contact with the recording material, and the coloring material is heated with a thermal recording head and selectively transferred to the recording material to obtain a desired image.
According to this recording system, the coloring material on the support is melted, evaporated, and sublimated by the application of heat. It is then transferred to the recording material to form an image through sticking, adsorption, and dye-fixing. One of the admitted characteristics of this recording system is that plain paper (non-coated paper) may be used as the recording material. In particular, a system using a sublimable dye as the coloring material provides an image excellent in image gradation, and hence attempts have been made to apply this system to full color recording.
However, when plain paper is used as the recording material, the dye-fixing, in particular, is difficult to accomplish. As a result, not only is the resulting recorded image low in color density, but serious fading occurs with time. Therefore, it has been suggested to use an image-receiving sheet having an image-receiving layer mainly comprising a thermoplastic resin as described in JP-A-57-107885, JP-A-59-165688, and U.S. Pat. No. 3,601,484.
Although image-receiving layer mainly comprising a thermoplastic resin achieves improvements on recording sensitivity and storage stability to some extent, there still remain some problems to solve. For example, the recorded image fades when irradiated with light, or undesired heat fusion between the image-receiving sheet and the coloring material-transferring layer takes place.
In order to prevent such undesired heat fusion between the image-receiving sheet and the coloring material-transferring layer, it has been recommended to incorporate a release agent into the image-receiving layer as disclosed in JP-A-60-34898 and JP-A-60-212394 or to provide a release layer on the image-receiving layer as disclosed in JP-A-59-165688.
However, an image-receiving sheet containing liquid or semi-solid silicon compounds, fluorine compounds, waxes, higher fatty acids, or higher fatty acid salts as the release agent exhibits deteriorated storage stability after image formation so that the image would undergo bleeding. When, in particular, a plurality of coloring material-transferring sheets having different colors, such as yellow, red, blue, and black, are successively used for obtaining a color image, the resulting image is poor in color reproduction, or undesired heat fusion between the image-receiving sheet and the coloring mateial-transferring layer is apt to occur as the recording is successively carried out. In cases where reaction-curable silicon compounds are used as the release agent, the image-receiving layer coated with the release agent should be subjected to curing by heating or irradiation of actinic energy rays, thus giving rise to a problem of low productivity. In addition, use of these release agents reduces recording sensitivity.